The Phnom Penh Municipal Court convicted Sam Rainsy, head of the Cambodia National Rescue Party, of defaming Heng Samrin, a legislator and senior member of the Cambodian People's Party.

The court ordered Sam Rainsy to pay 150 million riel ($37,500) compensation to Heng Samrin and a 10 million riel ($2,500) fine. He had claimed that Heng Samrin, while Cambodia's president from 1979 to 1992, had accused the country's then-royal leader, Norodom Sihanouk, of being a traitor.

Sam Rainsy has been abroad since mid-November, when an order for his arrest was issued on an old conviction for defaming former foreign minister Hor Namhong.

It had been generally assumed that the conviction, carrying a two-year prison sentence, had been lifted by a 2013 pardon. At that time, Sam Rainsy said in a Facebook posting that "the Cambodian authorities use a well-oiled machinery of justice for political purposes."

Prime Minister Hun Sen's government last year began cracking down on the opposition through the court system, which is considered a tool of the ruling party.

In another legal action in late November, a summons was issued for Sam Rainsy to hear charges of falsifying public documents, using fake documents and inciting social unrest. It apparently related to Facebook postings by a fellow member of his Cambodia National Rescue Party of mistranslated documents about the border with Vietnam. Allegations that the government is sympathetic to Vietnam, Cambodia's traditional enemy, are a volatile political issue.

Deputy opposition leader Kem Sokha is also facing several legal actions that make him vulnerable to arrest, and he has been holed up at his party's headquarters for weeks. Both he and Sam Rainsy alleged that the charges against them are specious and politically motivated.

Other party members and non-governmental workers also have come under legal and even physical attack.

The opposition posed a stronger than expected challenge against Hun Sen's ruling party in the 2013 general election, and the moves against it are seen as preparation for local elections next year and the next general election in 2018.